When it comes to historical linguistics (history, prehistory, Indo-European studies, etymology) of Latin and Greek, what are the most important resources? The resources can be historical grammars, etymological dictionaries, or anything similar. I am not looking for a full bibliography, but useful reference material.
When you suggest a source, please indicate how it is available (online, in print, free, paid), what it covers, and what makes it useful in your opinion.
Please give only one resource per answer. If you have many resources to suggest, give multiple answers — but read the other answers to avoid duplication. This way people can vote on individual resources, causing the best ones to rise up and the worst ones to sink down, so that we can easily find the most useful resources. If there are many resources listed in a single answer, it is hard to link the answer score to the quality of each listed item.
Note about resource questions:
Broad resource questions like this are generally not allowed on this site. The community chooses a select few to avoid an overflow of questions of this kind. If you have an opinion on what resource questions should be asked next, post a suggestion in this meta post or vote the existing ones. The ones to be asked will be chosen from that list according to their number of votes. If you have ideas about listing resources on this site in general, you can contribute to this meta discussion.
In particular, if you would like to ask a similar resource request question, do not just ask it, but post a suggestion in the mentioned meta post instead.